1. Field of the Invention
The present invention relates generally to wireless communication systems and, in particular, to forward link power control during soft handoff in wireless communication systems.
2. Description of the Related Art
Wireless communication systems employ Code Division Multiple Access (xe2x80x9cCDMAxe2x80x9d) modulation techniques to permit a large number of system users to communicate with one another. Such systems work because each signal is coded with spreading sequences, such as with a pseudo-random noise (xe2x80x9cPNxe2x80x9d) sequence, and orthogonal spreading sequences, such as Walsh codes. This coding permits signal separation and signal reconstruction at the receiver. In typical CDMA systems, communication is achieved by using a different spreading sequence for each channel. This results in a plurality of transmitted signals sharing the same bandwidth. Particular transmitted signals are retrieved from the communication channel by despreading a signal from all of the signals. Despreading is achieved by using a known user despreading sequence related to the spreading sequence implemented at the transmitter.
FIG. 1 illustrates CDMA system 100. The geographic area serviced by CDMA system 100 is divided into a plurality of spatially distinct areas called xe2x80x9ccells.xe2x80x9d Although cells 102, 104, 106 are illustrated as a hexagon in a honeycomb pattern, each cell is actually of an irregular shape that depends on the topography of the terrain surrounding the cell. Each cell 102, 104, 106 contains one base station 112, 114, and 116, respectively. Each base station 112, 114, and 116 includes equipment to communicate with Mobile Switching Center (xe2x80x9cMSCxe2x80x9d) 120, which is connected to local and/or long-distance transmission network 122, such as a public switch telephone network (PSTN). Each base station 112, 114, and 116 also includes radios and antennas that the base station uses to communicate with mobile terminals 124, 126.
When a call is set up in CDMA system 100, mobile terminal 124 communicates with the base station from which mobile terminal 124 receives the strongest pilot signal, in this case base station 112. Base station 112 and mobile terminal 124 communicate over a forward link and a reverse link. The forward link includes communication channels for transmitting signals from the base station to the mobile terminal, and the reverse link includes communication channels for transmitting signals from the mobile terminal to the base station. Base station 112 transmits control information to mobile terminal 124 over a communication channel, referred to herein as a forward control channel, and it transmits voice or data over a communication channel, referred to herein as a forward traffic channel. Mobile terminal 124 transmits control information to base station 112 over a communication channel, referred to herein as a reverse control channel, and it transmits voice or data over a communication channel, referred to herein as a reverse traffic channel. The communication channels are organized in 20-millisecond (ms) time periods, referred to herein as frames. Forward traffic frames are frames transmitted over the forward traffic channel, and reverse traffic frames are frames transmitted over the reverse traffic channel.
The number of signals that can be transmitted simultaneously is limited by each of the transmitted signals"" fraction of the total power, referred to herein as the power fraction. Thus, reducing the power fraction of each of the signals increases the capacity of the wireless communication system. However, reducing the power fraction of a signal increases the number of errors in that signal. A goal of power control is to adjust the power level of the signals in such a way as to keep the power fractions as close as possible to a level that allows the system to maximize capacity while keeping the number of errors in the signal at an acceptable level. Forward link power control varies the power output of the base station to maintain a constant frame error rate at the mobile terminal. A frame error occurs when one or more uncorrectable bit errors occur in a frame. The frame error rate is the number of frame errors divided by the total number of frames observed. A targeted frame error rate, typically between 1% and 3%, depending on the desired system performance, is selected to minimize power without compromising signal quality. If the frame error rate exceeds the targeted frame error rate, the usefulness of the signal is reduced and the power level is increased to decrease the number of frame errors. If the frame error rate is below the targeted frame error rate, the power level exceeds the optimum power level, and the power level is reduced.
When the mobile terminal is in a soft handoff, all the base stations involved in the soft handoff are involved in the forward link power control. When mobile terminal 126 receives fairly strong pilot signals from more than one base station, in this case from three base stations 112, 114, and 116, the mobile terminal is in soft handoff. This typically occurs when mobile terminal 126 is close to the edge of a cell. All three base stations 112, 114, and 116 transmit control information to mobile terminal 126 over respective forward control channels, and voice or data over respective forward traffic channels. In soft handoff, mobile terminal 126 transmits control information to all three base stations 112, 114, and 116 over respective reverse control channels, and it transmits voice or data to all three base stations 112, 114, and 116 over respective reverse traffic channels.
Base stations 112, 114, and 116 transmit forward traffic frames. Each forward traffic frame includes voice or data and error control information, typically in the form of a cyclical redundancy code (CRC). By contrast, each reverse traffic frame includes voice or data and error indicator bits (EIB) for indicating whether the last received forward traffic frame contained an error. Mobile terminal 126 receives the transmissions from all three base stations 112, 114, and 116 and combines the signals from all three to obtain the forward traffic frame. Mobile terminal 126 then checks the CRC of the combined signal to determine whether the forward traffic frame is in error. Mobile terminal 126 indicates this determination to all three base stations 112, 114, and 116 using the EIB in the next reverse traffic frame that mobile terminal 126 transmits. For example, a zero error indicator bit indicates that the forward traffic frame is not in error, and a positive error indicator bit indicates the forward traffic frame is in error. Upon receiving reverse traffic frames from the mobile terminal, the base stations sends the EIB to selection distribution unit (SDU) 128. SDU 128 examines all three EIBs, and determines whether the majority of the EIBs indicate an erred forward traffic frame. SDU 128 then indicates to all three base stations whether, and how, they should adjust the power of their forward links. For example, mobile terminal 126 can send an EIB indicating an erred forward traffic frame. Base station 112 and 116 can receive EIB that indicating that there is an error in the frame. However, due to interference on the reverse traffic link between mobile terminal 126 and base station 114, base station 114 receives an EIB indicating that the frame is not erred. After receiving and examining all three EIBs, SDU 128 would determine that there is an erred frame and indicate to all three base stations to increase the power of their forward link. Typically, it takes about five frames for the base station to transmit the EIBs to the SDU, and for the SDU to perform the determination and notify the base stations.
Therefore, in a conventional CDMA wireless communications system during soft handoff, there is a five frame, i.e., 100 ms, delay between the reception of the current power control information and the power control decision based on the information. In CDMA 2000 wireless communications systems the speed of power control is 800 Hz. Each frame includes sixteen 1.25 ms time intervals, referred to herein as power control groups. Power control information, referred to herein as a power-control bit, is sent every 1.25 ms, or once every power control group. Therefore, during the 100 ms delay in the power control decision, each base station receives new power control information 80 times. By the time the SDU indicates to the base stations how to adjust the power on the forward link, the information on which the SDU based this decision has been updated so many times that it just as likely to be incorrect as to be correct. Making the decision on 100 ms-old information loses much of the benefit of providing power control information every 1.25 ms.
Eliminating the step of sending the power control information to the SDU and then back to the base stations by performing the power control decisions at the base station allows the power control information to be used before it is outdated. However, it presents another serious problem. As described above, the three base stations can receive different power control information due to interference and fading on the reverse link. Therefore, the power level of some base stations will go up and the power level of other base stations will go down, causing a divergence between the power levels on the forward link of these base stations. The mobile terminal receives the strongest signal form one of the base stations, referred to herein as the primary base station, and weaker signals from other base stations, referred to herein as the secondary base stations. To ensure that the primary base station sends the signal at a large enough power for the signal to be received without too many errors, the secondary base stations may produce too much power. When the secondary base stations produce too much power, their capacity is reduced, which reduces the capacity of CDMA system 100. This problem is further exacerbated when the base station with the strongest forward link is not the base station with the strongest reverse link.
FIGS. 1 and 2 illustrate this problem in more detail. FIG. 2 illustrates the power level of the traffic channel over time. At time T all three base stations 112, 114, and 116 are at a particular power level, P. Mobile terminal 126 transmits a power control bit to raise the power of the forward link. Base stations 112 and 116 receive a power control bit requesting the base station to increase their power, therefore they increase the power level of the forward link by a step size. However, due to interference on the reverse traffic link between mobile terminal 126 and base station 114, base station 114 receives a power control bit requesting it to decrease the power, therefore it decreases the power level of the forward link by a step size. Because base station 114 had the strongest forward link and it just reduced the power level of the forward link, mobile terminal 126 is still not getting the signal at a desired power. Mobile terminal 126 sends another power control bit requesting that the base stations increase the power on the forward link. If the reverse traffic link does not improve, base station 114 can again receive an incorrect power control bit while the other base stations receive the correct power control bit. This lowers the power level of the forward link from base station 114, and raises the power level of the forward links from base stations 112 and 116. Mobile terminal 126 again sends a power control bit requesting that the power on the forward link be increased.
When, at T+2.5, base station 114 finally receives the correct power control bit, it increases the power level 130 on its forward link. This is repeated until T+5, when mobile terminal 126 finally receives the signal at an acceptable power level. Base stations 112 and 116 also receive the power control bits to increase the power level, and also increase the power level 140 and 150 on their forward links. These two base stations 112 and 116 are now producing a great deal more power than necessary, which reduces the capacity of these two base stations and, therefore, reduces the capacity of CDMA system 100.
Accordingly, there exists a need for controlling power quickly while reducing the divergences between the power levels of the several base station in a soft handoff.
The invention solves the above problems by programming each base station in a soft handoff with a threshold power level to control the power transmitted by the base station on the forward link. When the threshold power level is a minimum-threshold power level, each base station maintains its transmit power level at or above the minimum-threshold power level. When the threshold power level is a maximum-threshold power level, each base station maintains its transmit power level at or below the maximum-threshold power level. Preferably, all power levels are expressed in decibels (dB) relative to the pilot""s power level.
The threshold power level can be an adjustable threshold power level or a fixed threshold power level. In the case where the threshold power level is fixed, each base station is programmed with the fixed threshold power level, and each base station decides how to adjust its transmit power level locally based on the fixed threshold power level without input from other base stations.
In the case where the threshold power level is adjustable, the threshold power level is adjusted by a threshold step size when the transmit power level is substantially equal to the threshold power level for at least a predetermined percentage of a time period. For example, the threshold power level is adjusted by a threshold step size when the transmit power level is substantially equal to the threshold power level for at least 50% of the power control groups of a frame. There are two alternatives on how each base station decides how to adjust its transmit power level. In one alternative, each base station participating in a soft handoff sends its power control information to a processor which adjusts the threshold power level and notifies each base station of the new adjusted threshold power level. In the meantime, each base station uses the threshold power level it currently has to locally adjust its transmit power level. In the other alternative, each base station adjusts the threshold power level locally without input from other base stations, and uses the threshold power level to adjust its transmit power level.